Switch to Append style encoding

Signed-off-by: Alun Evans <[email protected]>

marshalers.go

@@ -10,6 +10,7 @@ import (
 	"github.com/cilium/ebpf/internal"
 	"github.com/cilium/ebpf/internal/sys"
 	"github.com/cilium/ebpf/internal/sysenc"
+	"golang.org/x/exp/slices"
 )
 
 // marshalMapSyscallInput converts an arbitrary value into a pointer suitable
@@ -43,33 +44,36 @@ func makeMapSyscallOutput(dst any, length int) sysenc.Buffer {
 	return sysenc.SyscallOutput(dst, length)
 }
 
-// marshalSlice encodes a slice containing one value per
+// appendPerCPUSlice encodes a slice containing one value per
 // possible CPU into a buffer of bytes.
 //
 // Values are initialized to zero if the slice has less elements than CPUs.
-func marshalSlice(slice any, elemLength, possibleCPUs, alignedElemLength int, buf []byte) error {
+func appendPerCPUSlice(buf []byte, slice any, possibleCPUs, elemLength, alignedElemLength int) ([]byte, error) {
 	sliceType := reflect.TypeOf(slice)
 	if sliceType.Kind() != reflect.Slice {
-		return errors.New("per-CPU value requires slice")
+		return nil, errors.New("per-CPU value requires slice")
 	}
 
 	sliceValue := reflect.ValueOf(slice)
 	sliceLen := sliceValue.Len()
 	if sliceLen > possibleCPUs {
-		return fmt.Errorf("per-CPU value exceeds number of CPUs")
+		return nil, fmt.Errorf("per-CPU value exceeds number of CPUs")
 	}
 
+	// Grow increases the slice's capacity, _if_necessary_
+	buf = slices.Grow(buf, alignedElemLength*possibleCPUs)
 	for i := 0; i < sliceLen; i++ {
 		elem := sliceValue.Index(i).Interface()
 		elemBytes, err := sysenc.Marshal(elem, elemLength)
 		if err != nil {
-			return err
+			return nil, err
 		}
 
-		offset := i * alignedElemLength
-		elemBytes.CopyTo(buf[offset : offset+elemLength])
+		b := make([]byte, alignedElemLength)
+		elemBytes.CopyTo(b) // Only copies elemLength
+		buf = append(buf, b...)
 	}
-	return nil
+	return buf, nil
 }
 
 // marshalPerCPUValue encodes a slice containing one value per
@@ -83,8 +87,9 @@ func marshalPerCPUValue(slice any, elemLength int) (sys.Pointer, error) {
 	}
 
 	alignedElemLength := internal.Align(elemLength, 8)
-	buf := make([]byte, alignedElemLength*possibleCPUs)
-	if err := marshalSlice(slice, elemLength, possibleCPUs, alignedElemLength, buf); err != nil {
+	buf := make([]byte, 0, alignedElemLength*possibleCPUs)
+	buf, err = appendPerCPUSlice(buf, slice, possibleCPUs, elemLength, alignedElemLength)
+	if err != nil {
 		return sys.Pointer{}, err
 	}
 
@@ -108,18 +113,15 @@ func marshalBatchPerCPUValue(slice any, batchLen, elemLength int) (sys.Pointer,
 	}
 
 	alignedElemLength := internal.Align(elemLength, 8)
-	stride := alignedElemLength * possibleCPUs
-	result := make([]byte, batchLen*stride)
-	buf := result
+	buf := make([]byte, 0, batchLen*alignedElemLength*possibleCPUs)
 	for i := 0; i < batchLen; i++ {
 		batch := sliceValue.Slice(i*possibleCPUs, (i+1)*possibleCPUs).Interface()
-		if err := marshalSlice(batch, elemLength, possibleCPUs, alignedElemLength, buf[:stride]); err != nil {
+		buf, err = appendPerCPUSlice(buf, batch, possibleCPUs, elemLength, alignedElemLength)
+		if err != nil {
 			return sys.Pointer{}, fmt.Errorf("batch %d: %w", i, err)
 		}
-
-		buf = buf[stride:]
 	}
-	return sys.NewSlicePointer(result), nil
+	return sys.NewSlicePointer(buf), nil
 }
 
 // unmarshalPerCPUValue decodes a buffer into a slice containing one value per